That door only opens after a certain point, so you didn't technically miss it. I think it's after your third boss fight? Which is usually Bloodstarved Beast

The creepy guys with staffs are called Doctors, and I think they give everyone trouble to start out with. Their timing may be a little different. I still take hits from them once in a while. But they're also pretty slow so it's not hard to rally your health back by immediately attacking them after

On that note though, it wasn't common knowledge when the game first came out, but just to let you know, when you kill an enemy, they have their death animation before they fall down, and you can continue to rally back health during that animation. So feel free to beat up the dead as they are dying if you are still missing health that you can get back

The hammer is a great weapon, it rips up enemy poise, but being so slow it takes a bit more finesse in its timing. But having a straight sword option is great as it's such a versatile weapon. You get a couple of them in the game, and it's such a reliable alternative to a main weapon

And yeah, Amelia seems a lot different from Cleric and Bloodstarved. She always gave me trouble with my first times with each character. But you eventually get used to it. Again, you wanna get up her butt, and try not to get too concerned when you can't see your character through all the fur lol..